Bipolar electrode

ABSTRACT

An improved dimensionally-stable bipolar electrode for use in electrochemical processes comprising a valve metal layer, suitable anodic material on the anode side of the valve metal, a barrier layer of germanium on the cathode side of the valve metal protected by a layer of common metal cathodic material. Such electrodes function at low hydrogen permeability rates during use in electrolytic processes.

United States Patent Schultz et al.

BIPOLAR ELECTRODE Inventors: Robert F. Schultz, Niagara Falls;

Edward H. Cook, Jr., Lewiston, both of NY.

Assignee: Hooker Chemicals & Plastics Corporation, Niagara Falls, NY.

Filed: May 25, 1973 Appl. No.: 363,927

US. Cl. 204/290 F; 29/196; 29/199 Int. Cl B0lk 3/04 Field of Search204/290 F References Cited UNITED STATES PATENTS 12/1966 Hall et a1204/290 F 4/1969 Colman 204/290 F 1 Apr. 15, 1975 Anthony 204/290 FRaetzsch et a 204/256 Primary ExaminerEdward J. Meros AssistantExaminer-Wayne A. Langcl Attorney, Agent, or FirmPeter F. Casella;Donald C. Studley [57] ABSTRACT 10 Claims, No Drawings BIPOLAR ELECTRODEThis invention relates to electrodes for use in electrolytic cells. Moreparticularly. this invention relates to improved. corrosion-resistant.dimensionally-stable bipolar electrodes particularly useful in theelectrolysis of alkali metal chlorides in the production of alkali metalchlorates.

The electrolysis of aqueous solutions of alkali metal chlorides such assodium chloride and potassium chloride has been conducted commerciallyfor years on a wide scale.

Graphite electrodes have been employed in the past in various alkalinechloride electrolysis operations: however. there have been certaindisadvantages which have arisen as a result of the use of graphite. Oneof the most serious of the disadvantages is the constant attrition ofthe graphite during the electrolysis operation. Attrition results in theincrease of the clearance or spacing between the anode and the cathodewhich in turn causes an increase in the cell voltage drop and aresulting decreasing efficiency in cell operation.

Graphite anodes have a limited life. generally being on the order ofabout 1.0 inches in thickness on installation but at the end of 10-12months ofcontinuous use may be reduced to about 0.25 inches. with theattendant loss in power and efficiency. Such losses. including economiclosses. have resulted in the proposed use of metallic electrodes and theuse of bipolar cells.

Generally in the production of alkali metal chlorate. bipolar electrodesare now preferentially used which. when arranged suitably in anelectrolytic cell. in a spaced electrical series. serve to function bothas the anode and cathode in the cell. The electrodes are subjected to anelectrical potential while immersed in the alkali metal chloridesolution and. electrochemically. alkali metal chlorate is produced.either in the cell itself. or outside the cell upon the standing of thesolution.

The advantages of the use of the bipolar cells and bipolar electrodesinclude:

a. bipolar cells are relatively simpler and more economical to producethan are monopolar cells;

b. the electrical contact for supplying current to the electrodes inbipolar cells is applied only through the first and last plates whilethe current supply to the anodes of monopolar cells must be supplied byelectrical contact established with each individual anode;

c. bipolar cells allow for the use of minimal distances between theelectrodes which reduces voltage and allows for a reduction of thevolume of electrolyteused.

Platinum group metal-coated electrolytic valve metals such as titaniumhave been proposed as substitutes for graphite anodes. The metallicelectrodes have offered several potential advantages over conventionalgraphite electrodes. as for example. lower over-voltage. lower erosionrates and the resulting electrolytic production of higher purityproducts. The economic advantages gained by the use of such electrodes.however. must be sufficiently high to overcome the high cost of thesemetallic electrodes.

A problem existant with bipolar electrodes based on anodic preciousmetals is that the titanium or valve metal support is attacked byhydrogen during the electrolysis on the cathode side. forming hydridesand causing disintegration of the electrode.

The present invention provides a bipolar electrode having excellentelectrolytic use characteristics and excellent durability. Theelectrodes of the present invention are composite bipolar electrodeshaving a base layer of valve metal. preferably titanium. an anodicmaterial. preferably a platinum group metal or metal oxide. deposited onone side ofthe valve metal. and a barrier layer of germanium on theopposing side of the valve metal. overlaid with a layer of suitablecathodic metal.

The electrode comprises a central or inner layer of a valve metal ofwhich the oxide is chemically resistant under anodic conditions to theelectrolyte employed. The expression "valve metal as employed herein isdefinitive of a metal which can function generally as a cathode in anelectrolytic cell. but not generally as an anode. due to the formation.under anodic conditions. of the oxide of the metal. which oxide oncedeveloped is highly resistant to the passage therethrough of electrons.

The preferred valve metal is titanium. although tantalum. or colombiummay also be advantageously employed.

The expression chemically resistant under anodic conditions"hereinbefore employed. as applied to the valve metal. indicates that theoxide is resistant to the corrosive surrounding electrolyte and is not.to an appreciable extent subject to erosion. deterioration ot toelectrolyte attack.

The germanium has a low hydrogen diffusion rate and effectively preventsthe migration of cathodically produced hydrogen from reaching thesurface of the valve metal.

One face of the central or inner valve metal layer is adhered to a layerof germanium. The valve metal may be adhered to the germanium layer byany means readily available in the art. particularly by sputtering thegermanium onto the valvemetal. The thickness of the layers is notcritical. it only being necessary that the thickness of the titanium orother valve metal central or inner layer furnish a self supportinglayer. and that the layer of germanium be of such thickness andcharacterisitcs as to provide an essentially hydrogen impermeablebarrier layer. Generally. the valve metal layer is on the order of fromabout 0.0] inch to about 0.70 or 0.80 inches in thickness. with thelayer of the germanium being on the order of from about 0.01 inch inthickness to up to about 0.1 inch in thickness. if desired.

At least an operable portion of the opposing face of the central orinner layer has bonded thereto a layer of suitable anodic materialchemically resistant under anodic conditions to the electrolyte used.The term suitable anodic material as employed herein refers to amaterial which is electrically conductive. resistant to oxidation andsubstantially insoluble in the electrolyte. Platinum is the preferredanodic material; however. it is also possible to utilize ruthenium.palladium. osmium. iridium. oxides of these materials. alloys of two ormore of the metals. or suitable mixtures thereof.

The cathodic material. or outer cathode side layer. may be of anysuitable. common cathodic material chemically resistant or insoluble inthe electrolyte under cathodic conditions. Such materials as steel.copper. chromium. cobalt. nickel. iron. or alloys of these metal may beused. The cathodic material may be applied to the germanium layer by anysuitable means known to the art which is nondestructive to the germaniumlayer. e.g.. electroplating. vacuum deposition. metal spraying or thelike. The thickness of the cathodic material layer may vary. but thematerial is generally utilized as a thin film. about 0.01 inch inthickness or greater.

The anodic material. preferably platinum. can be applied to the anodicside ofthe valve metal by using such sources of platinum aschloroplatinic acid or a thermally-decomposable metallo-organic compoundsuch as a platinum resinate.

For example. the metallic resinate may be mixed with an organic solventor diluent. such as terpenes or aromatics. typically oil of turpentine.xylene or toluene. before being applied to the base member. Theelectrode is heated to decompose and/or to volatilize the organic matterand other non-metallic components. leaving on the base member a layer ofadherent electroconductive platinum. ln producing a metallic anodiccoating by such method. care should be taken to avoid oxide formation.for example. by limiting the temperatures of heating or by effectingheating in an oxygenfree atmosphere such as in a vacuum or under anitrogen or argon blanket.

Heating may be effective in an air atmosphere: however. temperaturesabove about 600-650C are not recommended due to the possibility ofoxidizing the valve metal.

In the production of an anodic oxide coating. the temperatures and timeof heating are selected that will result in the formation of an oxide.preferably an oxide of a metal of the platinum series of metals. such asruthenium. The temperature applied may vary depending upon theparticular platinum metal used. Typically. the temperature may be in therange of from about 300 to about 600C. preferably from about 350C toabout 550C. with such temperatures applied for periods on the order offrom about 10 minutes to about 2 hours. The heating of the metal is mostadvantageously conducted in an atmosphere containing elemental oxygensuch as air or other oxygen-inert gas mixtures although an atmosphere orpure oxygen could be used. The platinum group metal oxide formed iseither crystalline or amorphous depending upon the temperature ofheating. with the degree of crystallinity increasing as temperatures andduration of heating are increased. Both crystalline. particularly if thecrystals are small in size. and non-crystalline coatings have goodelectroconductivity. Where the coatings have a low degree ofcrystallinity. improved adhesion and conductivity are noted.

It is not necessary that the anodic material be applied in such a manneras to completely cover the entire surface of the valve metal central orinner layer. However.

i the total anodic side of the central or inner layer should be coatedwith the anodic material to the extent that the massed portion of theanodic material function effectively as an anode. It is preferred thatthe anodic material essentially cover the anodic side of the valve metallayer.

The anodic layer. preferably a platinum group metal or metal oxide. canbe deposited to the extent of 0.0001 inch. although the use of lesser orgreater thicknesses may be achieved. depending on the methods ofdeposition. it only being necessary that the anodic material be presenton the anodic side of the central or inner layer in an amount sufficientto function effectively as the anode.

The anodic material. as hereinbefore stated. can be deposited on thecentral or inner layer of valve metal by any suitable method known tothe art. The deposition can be effected. for example. by using a bathconsisting of 4.5 grams platinic chloride and 22 ml 37 percenthydrochloric acid dissolved in 2.800 ml water. The temperature isgenerally maintained between 70 and 85C and the current intensity issuch that essentially no hydrogen is evolved at the valve metal panel. Agraphite anode is used in the bath and the valve metal is made cathodic.The panel is agitated or moved during the plating operation. and thecurrent is regulated as to preclude hydrogen involvement at the valvemetal. with the anodic platinum metal being deposited at a thicknessless than about 0.0001 inch. Minor varia tions may be effected in thedeposition of the precious metal and varying thicknesses may be obtainedby suitable modifications in the time consumed in the electroplatingoperation. Also. simultaneous deposition may be made of more than onecomponent. as for example. by effecting a coating from a solutioncontaining. in addition to platinum. another platinum group metalcompound such as ruthenium. such other component being added to theelectroplating bath whereby a desired re sulting composite coating isobtained by the electrode position.

The electrodes of the present invention. as stated. find particularapplication in the electrolytic production of alkali metal chlorates. lnproducing chlorates using the electrodes of the present invention. theprocess may be carried out continuously by passing a solution containingalkali metal chloride through the cells at temperatures generally on theorder of up to the boiling point of the electrolyte with the effluentliquor cooled or concentrated to promote crystallization of the chlorateproduced in the cell. Advantageously. a small amount of chromate may beadded to the liquor fed to the cell in order to promote chlorateformation. in accordance with methods known in the art.

A typical bipolar electrolytic unit which can be used with the novelelectrodes of the present invention consists of a housing havingspaced-apart end electrodes with the enclosed space defined by the wallsand end electrodes divided intermediate at intervals by the bipolarclectrodes into substantially isolated unit cells. For each individualelectrolysis cell. there is an individual reaction zone and anindividual electrolysis zone substantially isolated from the reactionand electrolysis zones of the adjoining cells. the term "unit cell"referring to one of the chambers or sections into which the apparatus isdivided by the bipolar electrodes. Such cell makeup permits of goodcirculation of the electrolyte between the zones.

A bipolar electrolytic cell utilizing the bipolar. electrodes describedhas essentially minimal or no current leakage and voltages on the orderof 3.8 to 4.0 volts can be employed. with a current density of about 4amp/in? What is claimed is:

l. A bipolar electrode consisting of a core of a valve metal. at least aportion of the anodic surface of which is conductively covered by atleast one anodic material selected from platinum group metals andplatinum group metal oxides. and a barrier layer of germanium on thecathodic side of the valve metal core, at least a portion of theexterior surface of the layer of germanium being covered by at least onecathodic material the cathodic material is iron.

7. A bipolar electrode as defined by claim 1 wherein the anodic materialis platinum metal.

8. A bipolar electrode as defined by claim 1 wherein the cathodic metalis nickel.

9. A bipolar electrode as defined by claim 1 wherein the cathodic metalis iron.

10. A bipolar electrode as defined by claim 1 wherein the anodicmaterial is ruthenium oxide.

1. A BIPOLAR ELECTRODE CONSISTING OF A CORE OF A VALVE METAL, AT LEAST APORTION OF THE ANODIC SURFACE OF WHICH IS CONDUCTIVELY CVERED BY ATLEAST ONE ANODIC MATERIAL SELECTED FROM PLATINUM GROUP METALS ANDPLATINUM GROUP METAL OXIDES, AND A BARRIER LAYER OF GERMANIUM ON THECATHODIC SIDE OF THE VALVE METAL CORE, AT LEAST A PORTION OF THEEXTERIOR SURFACE OF THE LAYER OF GERMANIUM BEING COVERED BY AT LEAST ONECATHODIC MATERIAL SELECTED FROM IRON, COPPER, COBALT, NICKEL AND ALLOYSOF THESE.
 2. A bipolar electrode as defined by claim 1 wherein the valvemetal is titanium.
 3. A bipolar electrode as defined by claim 2 whereinthe anodic metal is platinum metal.
 4. A bipolar electrode as defined byclaim 3 wherein the cathodic material is iron.
 5. A bipolar electrode asdefined by claim 2 wherein the anodic material is ruthenium oxide.
 6. Abipolar electrode as defined by claim 5 wherein the cathodic material isiron.
 7. A bipolar electrode as defined by claim 1 wherein the anodicmaterial is platinum metal.
 8. A bipolar electrode as defined by claim 1wherein the cathodic metal is nickel.
 9. A bipolar electrode as definedby claim 1 wherein the cathodic metal is iron.
 10. A bipolar electrodeas defined by claim 1 wherein the anodic material is ruthenium oxide.